Control of polymer degradation rate

ABSTRACT

The rate of degradation of anionic water-soluble polymers in the presence of persulfate ion is reduced and controlled by the presence of divalent manganese ion. The polymer can be either cross-linked or uncross-linked. Useful polymers to which the invention is applied are anionic acrylamidecopolymers and carboxymethyl cellulose.

United States Patent [191 Podlas [451 May 21, 1974 CONTROL OF POLYMER DEGRADATION RATE [75] Inventor: Thomas J. Podlas, Newark, Del.

[73] Assignee: Hercules Incorporated, Wilmington,

Del.

[22] Filed: Nov. 8, 1972 [21] Appl. No.: 304,701

[52] US. Cl 106/169, 260/231 CM, 260/296 M [51] Int. Cl. C08f 45/24, C08b 21/26 [58] Field of Search 260/296 M, 231 CM; 4

[56] References Cited UNITED STATES PATENTS 3,240,736 3/1966 Mckennon 260/29.6

3,719,663 3/1973 Klug 260/231 Primary ExaminerWilliam H. Short Assistant Examiner-Peter F. Kulkosky Attorney, Agent, or Firm-William S. Alexander [5 7] ABSTRACT The rate of degradation of anionicwater-soluble polymers in the presence of persulfate ion is reduced and controlled by the presence of divalent manganese ion. The polymer can be either cross-linked or uncrosslinked. Useful polymers to which the invention is applied are anionic acrylamidecopolymers and carboxymethyl cellulose.

4 Claims, No Drawings 1 CONTROL F POLYMER DEGRADATION RATE The use of thickened or viscous polymer solutions has become Wide-spread in the oil industry for various secondary recovery and fracturing operations. Materials useful in such operations are copolymers based on acrylamide and an anionic copolymerizable comonomer. Such copolymers exhibit a good degree of water solubility and relatively small quantities thereof in solution lead to significant viscosity increases. The viscosity of such solutions per se is sufficient in many instances to accomplish the objective sought to be accomplished. In other cases, a greater viscosity or even a gel is required. In these situations a cross-linker can be added. Preferred materials for cross-linking are trivalent metal ions, in particular, chromium and aluminum. in either situation, it is also frequently desirable to be able, after a time, to decrease or destroy the viscosity of the solution, thus making it free flowing and easily removable from the formation.

In previous work it has been found that polymers of the type herein contemplated, either cross-linked or otherwise, can be degraded by means of a watersoluble persulfate. This method of breaking viscosity is extremely rapid, particularly at elevated temperatures such as might be found in an oil, bearing formation and is thus frequently not totally satisfactory in situations where the high viscosity solution or the gel structure must be maintained for an extended period, say a day or more.

in accordance with this invention, it has been found that the degradation rate ofthe thickened "solution under the influence of a persulfate oxidizer can be decreased and better controlled if there is included in the reactive mixture a small amount of water-soluble compound of divalent manganeseqln brief, the invention comprises a method of controlling the rate of degradation under the influence of a water-soluble persulfate oxidizer of an anionic water-soluble polymer which method comprises including in an aqueous solution of said polymer and said persulfate a water-soluble ionizable'compound of divalent manganese. The method of the invention is applicable to both cross-linked and uncross-linked polymers. v

The polymers to which the process is preferably applied are copolymers of acrylamide and an anionic, ethylenically unsaturated comonomer. The preferred comonomer is sodium acrylate or sodium methacrylate. The comonomer can constitute about 1 to 50 percent by weight of the total copolymer, preferably about 1 to percent. Copolymers of this type are well known articles of commerce. The invention is also applicable to certain carboxyalkylated cellulose ethers such as, e.g., carboxymethyl cellulose.

The persulfate employed in the invention can be any water-soluble inorganic persulfate. Preferred are the alkali metal persulfates such as sodium persulfate, lithium persulfate, and particularly potassium persulfate. Other water-soluble persulfates, e.g., ammonium persulfate can also be employed. Generally the persulfate is present in concentration of about 0.1 to percent by weight based on the amount of polymer present, although persulfate concentration, per se is not critical to this invention. What is important is the molar ratio of manganous ion to persulfate ion.

The mechanism by which the persulfate breaks the viscosity of the polymer solution is believed to involve breakdown of the polymer chain, undoubtedly via an oxidation reaction, although the point in the chain where the breakdown occurs is not known with certainty. Additionally, in the case of polymers which are cross-linked, e.g., with chromium ion, to form gels, the persulfate can oxidize the metallic cross-linking ion and thus destroy the cross-link and break the gel. The viscosity can be decreased by this technique virtually to that of water.

Any water-soluble, ionizable salt of divalent manganese can be employed in the process of this invention to control the rate of polymer degradation. Exemplary of such salts are manganous sulfate, manganous chloride, manganous nitrate, and manganous acetate.

The mechanism by which the manganous ion controls the degradation rate has not been definitely determined. However, it has been determined that divalent manganese ion can react with the ionic substituents of thecopolymer to cause some viscosity build-up, which is indicative of some cross-linking and which can slow the rate of degradation. Further, the divalent manganese ion can be oxidized by the persulfate to manganic ion and the competition between the polymer and the manganese for the persulfate in effect reduces the persulfate concentration, resulting in slower degradation. In addition, the resulting manganic ion can cross-link the polymers which also slows the degradation rate. Cross-links resulting from the divalent manganese or manganic ion are then degraded by heat or other influences acting on the system.

ln carrying out the process of this invention, it is preferred to prepare first a water solution of the copolymer and to add to this a solution containing a mixture of the manganous salt and the persulfate. Inasmuch as the persulfate can react with both of the other components in the system, it is desirable and even mandatory that all mixing of ingredients be effected immediately prior to utilization of the product, e.g., immediately prior to charging the viscous liquid to an oil-bearing formation. Alternatively, the manganous salt can be added to the polymer solution and, following dissolution thereof, a persulfate solution can be added to this mixture. In any event, there is-sufficient reactivity between elements of the mixture that any mixing thereof should be done substantially immediately prior to use.

The invention is illustrated in the following examples, in which, unless otherwise specified, parts and percentages are by weight.

EXAMPLE I using a FANN VG viscometer. To each of three aliquots of this' solution there was added 9 percent by weight, based on polymer, of potassium persulfate. One of the three was used as a control, without further modification, to one was added (based on polymer) 1.00 percent manganous sulfate and to the third there was added (based on polymer) 2.00 percent manganous sulfate. All three were then heat treated in an oven at 60C. and their rates of degradation were followed via periodic viscosity readings on a FANN VG viscometer. Results are expressed as the percentage of the original 3 ,81 1,902 3 viscosity which the solution retains after a given heat treatment time in the following table.

lowing table as the percentage of its initial viscosity wh ch i e in at va ious times- Example Mnso. Example N0. 1 hr. 2 hr. 3 1". 4 hr. 6 hr. 24 hrs.

No. Concentration i EL I I Mnr2/S O -3 Vlscoslty 2-A 82 22 5 0 Retentlon 243 84 77 71 65 54 0 2c 73 2 0 l-A 0 0 1.5 hrs. 58 2-D 81 73 67 59 45 0 l-B 1.00 0.20 do. 81 l-C 2.00 0.40 do. 82 '2' M N l-D 0 O 3 hrs. 145 1.00 0.20 do. 76 l-F 2.00 0.40 do. 78 l-G 0 0 6 hrs. s- 15 l-H 1.00 0.20 30. l-] 2.00 0.40 0. l-J 0 0 a 24 hrs. 0 EXAMPLE 3 Hi 188 8:38 281 in this example, the copolymer composition was l-M 0 0 48 l about 98.5 percent acrylamide and 1.5 percent sodium 1: 3 'gg 8'38 38' g acrylate and its concentration was 1 percent. The oven temperature was Viscosity Retention Exam %l ,s,0. vpl/lnso. Mn"/s,o,.-= 15 hr. 1 hr. 2 hr. 3 hr. 4 'hr. Y 5 hr. 20 hr. 24 hr. 48 hr ple No l 3-A 0.375 0 0 as 79 59 36 2.4 0 3-13 0.375 0.225 [.08 '88 a2 71 66 63 63 27 32 34: 0.625 0 0 84 7s 51 21 1.2 0 3-D 0.625 0.225 0.65 as so 71 68 63 63 l4.6 17.1 3-12 1.25 0 0 as 72 36 11 1.2 0 3-F 1.25 0.75 1.03 83 77 71 61 52 3s 6.1

XAMPLE 2 EXAMPLE 4 A one percent (1 percent) aqueous solutionof the polymer employed in Example 1 was prepared and di- In this example a 1 percent solution of the 90/10 covided into four aliquots. These were treated with manganous sulfate and/or potassium persulfate at the concentration levels shown in the chart below. When reapolymer used in Example -1 was treated at 80C. with 0.625 percent potassium persulfate and varying amou-- nts of manganous sulfate. The results, recorded in the gents were added, they were initially mixed in appropriate portions in dilute aqueous solution and this solution was added to the polymer solution immediately.

following table, indicate that the rate of viscosity loss can be controlled by varying the amount of manganous sul ate .rs'stiys E9919 amq l t 9 q tat w g Rei n Mn/S,O l r. r. v r. 4 hr. 5 hr.

Example No. EMnSO EXAMPLE 5 Example No. Mnso K,s,o. Mn"/s,0..-' A 1 percent aqueous solution of a copolymer of about 98.5 percent by weight acrylamide and 1.5 per- 0 0 60 cent sodium acrylate was prepared and divided into M 2 7% L04 several aliquots. To these was added 1.9 percent 2-c o 0.625% 0 LD 026% 0.625% (Q63 chrome alum to effect gelling and potasslum persulfate and manganous sulfate in the amounts shown in the following table. Gelling occurred within about 20 min 1 The four specimens thus prepared were heated in an utes. The gel were immediately placed in an C. oven and the time for gel breaking to occur was noted. The gel is considered to be broken when it becomes readily pourable without lumps.

Example No. %K,S,O,, %MnSO Mn/S,O,. Time for Gel Breaking 5-A 0.167 overnight S-B do. 0.5 5.4 Intact after 2 days 5-C do. 1.51 16.20 Intact after 2 days 5-D 0.33 34 hours -E do. 0.05 0.27 4-6 hours 5-F do. 0.1 0.54 4-5 hours 5-0 do. 0.2 1.08 Intact after 2 days 5-H do. 0.4 2.16 Intact after 2 days between 3 and 19 hours .m

EXAMPLE 6 It is clear from the data in the preceding examples that the presence of the manganese ion in the system Example 5 was repeated except that the copolymer Significantly delays the degradation of the P y n y dwas FPQQUPEQBQ XW will" Example 1 241999592 11 x 1 ises rl plss News s2 .9.

Example No. %K,S,O,. %MnSO Mf /S 0, Time For Gel Breaking s n 0.167 0 2-3 hours 6-B do. 0.5 5.40 overnighr' so do. 1.51 -.l6.2 do.

' between 7 and 24 hours.

EXAMPLE 7 simply dissolved to form a viscous liquid, The degree of A 1 percent solutionof a high viscosity carbon 25 control over degradation lS regulated by the molar ra'tlo methyl cellulose having D5. of 0.7 was preparedand of divalentmanganeseion to persulfate ion present in divided into three portions. To these were added 2.8 the systemh q h reflulred to f percent chrom alum, 9 percent o iu persulfate preselected viscosity cond tion varles nversely wlth the and 0, 0.1 percent and 0.2 percent respectively of man ratio of mangenese persulfate m h y ganous sulfate.- The solutions gelled almost instanta- In most cases, a mo ar ratio of manganese on to persulneously and were placed in an oven at 80C. until the. 'fate between about 0.02 and 20 will provide adequate gels broke as evidenced by their degradation to a freecontrol over the degradation. Higher ratios do not afflowing pourable liquid. Pertinent data are recorded in; f rd any b tt r n- L the following table- 5 What I claim and desire to protect by Letters Patent l. A method for controlling the rate of degradation Example %Mnso. Mn/s,o,,-* Time For Gel Breaking under the influence of a persulfate oxidizer of a watersoluble polymer selected from the class consisting of l) 0 0 40 copolymers of acrylamide andan anionic copolymeriz- -2 8; 8-83 2 32 able comonomer and 2) carboxymethylcellulose which method comprises incorporating, into an aqueous solubetween lfland 22 hours. tion of said polymer and said persulfate, a watersoluble salt of divalent manganese in such quantity that the molar ratio of manganese ion to persulfate ion is between about 0.02 and'20.

2. The method of claim 1 where the anionic water- EXAMPLE 8 soluble polymer is a copolymer of about 90 to 99 per- A 119 pqqgmig water was prepared with a 90/10 cent by weight acrylamide and 10 to 1 percent by rylamide sodium acrylate copolymer. To each of three we'ght sodlum acrylatealiquots of this solution'was added 0.625 percent potas- The method of claim 1 where the anionic water sium persulfate and varying amounts of manganous SQIuble polymer is carbPxymethyl cellulose chloride These were heated at in an oven 4. The method of 01am 2 where the salt of. d1valent cosity retention data are recorded in the following tamanganese is manganous Sulfate and the persulfate is ble. BQ l lT ,B!l!l -M 7% Viscosity Retention Example No. %MnCl, Mn =/s,o.-= 1 hr. 2 hr. 3 hr. 4 hr. 5 hr.

8-A 0 0 81 38 I6 8-B 0.15 0.45 85 74 71 55- 42 8-C 0.225 0.67 87 78 61 56 

2. The method of claim 1 where the anionic water-soluble polymer is a copolymer of about 90 to 99 percent by weight acrylamide and 10 to 1 percent by weight sodium acrylate.
 3. The method of claim 1 where the anionic water-soluble polymer is carboxymethyl cellulose.
 4. The method of claim 2 where the salt of divalent manganese is manganous sulfate and the persulfate is potassium persulfate. 